First you have to get an MPQ viewer. They have a really good one here on Hive, that can convert the .tga files into .blp. From there you convert the .blp to a .bmp (using Biturn, another program on Hive) if you just want to play around with it on Paint. Otherwise, if you have Photoshop, you don't need Biturn. You can just do this backwards when you have what you want.